The invention relates to treating wells by acidizing materials in and around their boreholes with a hot, weak acid. More particularly, the present invention relates to conducting such treatments with such an acid that is chemically heated at or near the zone being treated.
Numerous uses of self-reacting aqueous solutions of compounds containing nitrogen generating reactants have been described in patents and patent applications such as the following: U.S. Pat. No. 4,178,993 by E. A. Richardson and R. F. Sherman describes a well treating process for initiating fluid production by injecting an aqueous solution containing nitrogen-gas-generating reactants having a concentration and rate of reaction correlated with the pressure and volume properties of the reservoir and the well conduits to react at a moderate rate within the well and/or the reservoir to generate enough gas to displace sufficient liquid from the well to reduce the hydrostatic pressure within the well to less than the fluid pressure within the reservoir.
U.S. Pat. No. 4,219,083 by E. A. Richardson and R. F. Sherman describes a process for cleaning well casing perforations by injecting an aqueous solution containing nitrogen-gas-generating reactants, an alkaline buffer providing a reaction-retarding pH and an acid-yielding reactant which subsequently overrides the buffer and lowers the pH in order to trigger a fast-rising pulse of heat and pressure which causes perforation-cleaning backsurge of fluid through the perforations.
U.S. Pat. No. 4,232,741 by E. A. Richardson, R. F. Sherman, D. C. Berkshire, J. Reisberg and J. H. Lybarger describes a process for temporarily plugging thief zones within a reservoir by injecting an aqueous solution containing nitrogen-gas-generating reactants, a foaming surfactant, an alkaline buffer and an acid-yielding reactant, arranged so that they initially delay the reaction and subsequently initiate a moderate rate of gas production, in order to form a foam which is, temporarily, substantially immobile within the reservoir formation.
U.S. Pat. No. 4,330,037 by E. A. Richardson and W. B. Fair, Jr. relates to injecting an aqueous solution which contains a concentration of gas-generating reactants and reaction rate-controlling materials which are correlated with the rate at which the solution is injected into the well and reservoir so that they both heat a selected zone to a selected temperature and, concurrently, increase the relative magnitude of the effective permeability to oil within a treated portion of an oil and water-containing reservoir.
Commonly assigned patent application Ser. No. 127,355 filed Mar. 5, 1980, by D. R. Davies and E. A. Richardson describes a process for conducting a production test by circulating a solution of nitrogen-gas-generating reactants within conduits within a well, with the solution buffered at a pH providing a promptly-initiated reaction having a relatively mild rate, and being inflowed through a conduit at a rate such that the gas which it generates serves as a lift gas for gas-lifting fluid from the reservoir through another conduit within the well.
The disclosures of the above patents and application are incorporated herein by reference.
Numerous uses of acidizing solutions comprising weak acids and/or their salts are described in patents such as the following: U.S. Pat. No. 2,301,875 describes employing an acidizing medium for oil wells comprising an aqueous buffer solution composed of a weak acid and a salt of a weak acid. U.S. Pat. No. 2,640,810 describes including formic acid or one of its salts in a hydrochloric acid solution to prevent the pH of the acid solution from rising above about 4. U.S. Pat. No. 3,142,335 describes including citric acid and salts of low molecular weight organic acids in hydrochloric acid as a sequestering agent for iron or the like. U.S. Pat. No. 3,441,085 describes injecting a weak acid having a concentration exceeding that needed to form a saturated salt solution and then injecting water to solubilize the acid salts.